Compositor nodes missing
There is a second scene called testlight, which has Use nodes disabled.

Enable nodes in testlight, then the nodes will appear and work fine in the main scene.



I'm not sure why this happens. I think it might be a bug, you might want to try reporting it (particularly if you can figure out how it got this way in the first place).

Re-disabling use nodes in testlight doesn't seem to disable nodes in the other scene again.